1. Field of the Invention
This invention relates to apparatus for transporting a flexible sheet through manufacturing processes, and, more particularly, to apparatus for loading a flexible sheet onto a frame, for transporting the flexible sheet with the frame through the process and for removing the flexible sheet from the frame.
2. Background Information
Processes applied during the manufacture of flexible circuits, which are applied, for example, to polyimide sheets having a thickness of 0.002 inch (0.08 mm), require the holding and tensioning of such sheets as membranes having sides up to 14 inches (35.6 cm) long. Such circuits, and the sheets from which they are made, must be held as they go through one or more chemical process steps during manufacture.
Present methods for holding circuit sheets as they are carried through chemical processes include the use of double-sided adhesive tape, pins, springs, clamps, and screws. (Hereinafter, the term "circuit sheet" is meant to describe a flexible sheet being prepared for the application of circuit traces, a flexible sheet to which circuit traces are being applied, or a flexible sheet to which circuit traces have been applied). Double-sided adhesive tape used to fasten a flexible sheet or circuit to a frame member must frequently be replaced on the frame member, in an operation which is often difficult because the tape cannot easily be removed completely and cleanly. Furthermore, such tape frequently transfers an adhesive residue to the product being manufactured. Mechanical clamping methods tend to be complex, expensive to build, and difficult to maintain. Such mechanisms may collect and retain solutions from various chemical processes, resulting in a need for frequent and difficult cleaning, in corrosion problems, and, sometimes, in the contamination of one chemical process with residual materials from another such process. Furthermore, such conventional clamping mechanisms often require special features in the product being manufactured, such as clearance holes for clamping pins or screws. In some instances, difficulties are encountered because the thin sheet product is not clamped uniformly, being allowed to sag or billow between conventional clamping devices.
Thus, what is needed is a frame, to which a flexible sheet can easily be attached, and from which the circuit sheet can easily be removed, having a capability to maintain membrane tension within the circuit sheet as it is carried through various chemical processes. It is particularly desirable that such a frame be composed of materials resistant to chemical attack, and that such a frame be configured to avoid presenting surfaces including notches or cavities to the chemical processes through which the circuit sheet may be carried, since such features tend to collect chemical residue and are difficult to clean. It is further desirable to provide attachment means usable with a number of different types of circuit sheets without placing requirements on such sheets for special features, such as attachment holes. Such a frame is needed for the chemical processing of flexible circuit sheets and for the processing of other types of flexible sheets.
3. Description of the Prior Art
A work board for holding a flexible circuit flat to provide compatibility with automated, robotic assembly and handling during the manufacturing processes of building a populated working flexible circuit is described in Article 37535, RESEARCH DISCLOSURE, March, 1987, number 275, Kenneth Mason Publications Ltd., England. This work board includes a frame having two holes, spaced at mid-points along opposite sides, receiving pins extending through holes in the flexible circuit to locate the circuit. At each corner of the work board, a spring-loaded, low profile slider hooks into an additional hole in the flexible circuit. Thus, the circuit is tensioned by pulling all four corners as it is held in place at mid-points along opposite sides.
However, this work board includes a number of mechanical clamping devices, which must be cleaned, with resulting difficulties, after passage through various types of chemical baths employed in the process of manufacturing flexible circuits populated with components. Furthermore, six special holes must be placed in the circuit being manufactured for engagement with these clamping devices.
A different type of tensioning frame is described in U.S. Pat. No. 5,136,797. This frame assembly, which is used to hold the mesh fabric used for screen printing in a tensioned condition through a number of printing operations, includes adjustable tensioning bars adjacent the frame peripheral edge. Screen mesh fabric is mounted to the tensioning bars by adhesive, and the bars are drawn toward the frame edge by tightening screws extending through holes in the frame edge to engage the bars, In this way, the fabric is stretched to a uniform tautness for printing. Each tensioning bar may also include resilient corner pieces to allow local slackening of the fabric.
While this tensioning frame appears to work well for its intended purpose, it cannot be applied as a practical frame for holding a flexible circuit sheet as it is taken through the various chemical processes involved in the production of a flexible circuit. Each of the four sides of the frame comprises a metal channel, in which drive screws move bars with flexible corner pieces. This type of structure includes a number of places in which chemical residues can be easily trapped, causing the types of problems described above. In the printing process for which this tensioning frame is intended, ink is typically applied, using a squeegee, to the central portion of the mesh screen, along the image to be printed. There is little if any contamination by ink within the cavity formed by the frame channels.
U.S. Pat. Nos. 4,667,944, 4,711,041, and 4,778,326 describe apparatus for handling semiconductor chips and like objects. In U.S. Pat. Nos. 4,667,944 and 4,711,044, the object to be handled is placed on a flat flexible film that is supported on a flat, porous texturized porous texturized fabric sheet, such as a woven or knit fabric having spaced fiber crossovers. In U.S. Pat. No. 4,778,326, the object to be handled is attached to the face of a base member, with the face being formed with recesses covered by the flat flexible film. In the apparatus of all three patents, objects are supported on the flat flexible film in intimate surface contact therewith for securely holding the same in position by interfacial forces therebetween. Adhesive may be included for increasing the interfacial force. To facilitate removal of objects from the film, the fabric of U.S. Pat. Nos. 4,667,944 and 4,711,041 is connected to a vacuum source for drawing portions of the flexible film over and between crossovers in the fabric. Similarly, the recesses of the face of the base member described in U.S. Pat. No. 4,778,326 are connected to a vacuum source, drawing portions of the flexible film into the recesses and providing the film with a textured, or undulating surface. In any case, the application of a vacuum causes portions of the film to be withdrawn from the object being held, reducing the area of contact between the flexible film and the object being held, and hence the interfacial forces therebetween. In this way the removal of the object using a conventional technique is enabled.
While the methods of U.S. Pat. Nos. 4,667,944, 4,711,041, and 4,778,326 are particularly effective in facilitating the handling of small, flat devices, such as semiconductor chips, what is needed is an effective method for handling relatively large flexible sheets. Since the methods of these three patents are designed for small chips, a need to hold flexible sheets in a taut, tensioned condition is not addressed. It is also desirable to avoid the use of a vacuum system, with its ports and cavities, in the configuration of a frame being used to transport flexible sheets through chemical processes. An effective, automated system for attaching flexible sheets to frames holding such sheets in a tensioned condition, for transporting such frames with sheets attached therethrough through manufacturing processes, and for subsequently removing the sheets from the frames is also needed.